Fix ref and relref shortcode usage patterns

The documentname parameter to `ref` and `relref` shortcodes must include file extension. Including the file extension in the given usage patterns will make it much easier for users to follow.

content/content-management/cross-references.md

@@ -21,12 +21,12 @@ toc: true
 ## Use `ref` and `relref`
 
 ```
-{{</* ref "document" */>}}
+{{</* ref "document.md" */>}}
 {{</* ref "#anchor" */>}}
-{{</* ref "document#anchor" */>}}
+{{</* ref "document.md#anchor" */>}}
-{{</* relref "document" */>}}
+{{</* relref "document.md" */>}}
 {{</* relref "#anchor" */>}}
-{{</* relref "document#anchor" */>}}
+{{</* relref "document.md#anchor" */>}}
 ```
 
 The single parameter to `ref` is a string with a content `documentname` (e.g., `about.md`) with or without an appended in-document `anchor` (`#who`) without spaces.
@@ -120,4 +120,4 @@ Ensuring heading uniqueness across the site is accomplished with a unique identi
 [built-in Hugo shortcodes]: /content-management/shortcodes/#using-the-built-in-shortcodes
 [lists]: /templates/lists/
 [output formats]: /templates/output-formats/
-[shortcode]: /content-management/shortcodes/
+[shortcode]: /content-management/shortcodes/